Edit: I am a noob to this stuff. I used to have bitcoins when Mt.Gox was a thing but haven't used them since. Maybe some beginner mistake I made?

Some beginner's mistakes I can think of are:

Mistaking Bitcoins as BitcoinCash or BitCore.
Sending late to an invoice (temporary) address.
Misconfigured his wallet's Satoshi value (BTC, mBTC & μBTC) thus, sending the incorrect amount.
Sent BTC to a fake site.

But I'm going for this:

there's like a 5% chance the sending address was wrong but i really doubt it.

Wrong copy or "clipboard hijacking" incident.